ASMUSSEN, CONNY B.*, MARK W. CHASE, AND MICHAEL ZANIS. Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK. - Nuclear 26S rDNA and systematics of monocotyledons.
The most comprehensive phylogenetic study of monocotyledons based on
DNA sequences includes 100 of the 102 currently recognized families
(APG 1998) and is based on a combined analysis of rbcL, atpB, and 18S
(Chase et al. 2000). This phylogeny is well resolved and well
supported, except for the higher level inter-relationships among some
orders. Separate analyses of the plastid genes, rbcL and atpB, result
in well resolved strict consensus trees, whereas an analysis of the
nuclear 18S gene gives a far less resolved consensus tree.
Furthermore, the composition of a number of clades in the 18S topology
differs from those of the plastid gene topologies. It is therefore
important to add another nuclear DNA region to the three-gene data
set, to gain further confirmation for the plastid DNA topologies and
higher bootstrap support for the ordinal clades. Only a few studies
have used the 3400 base pair long 26S gene for cladistic analyses, and
we explore the phylogenetic information from this nuclear ribosomal
DNA region. The results presented are based on the first 1200 base
pairs, which contain six of the 12 expansion segments. These
sequences could be aligned easily across monocot families, except for
a few highly variable areas of the expansion segments. The 26S region
produced about the same number of parsimony informative characters as
rbcL and atpB, and more informative characters than 18S. The strict
consensus tree resulting from cladistic analysis of 26S sequences is
comparable to the well resolved strict consensus trees of rbcL and
atpB, and much better resolved than the 18S consensus tree. The 26S
topology largely recovered the clades present in the previous analyses
of rbcL, atpB, and combined rbcL, atpB, and 18S. Many smaller clades
received high bootstrap support, but most major clades had less than
50% support.
Key words: 26S nrDNA, molecular systematics, monocotyledons